From: "Ara.T.Howard" <Ara.T.Howard@...>
Date: 2005-09-11T04:01:55+09:00
Subject: Re: Mmap crash when appending

On Sun, 11 Sep 2005, Guillaume Marcais wrote:

> [gus@hibernatus Bifurcation]$ cat mmap-abort.rb
> require 'mmap'
> mm = Mmap.new("test-mmap", "w")
> 10.times { |i| p i; mm << "\0" * 1024 }
>
> [gus@hibernatus Bifurcation]$ > test-mmap
> [gus@hibernatus Bifurcation]$ ruby mmap-abort.rb
> 0
> 1
> 2
> 3
> 4
> mmap-abort.rb:3: [BUG] Bus Error
> ruby 1.8.2 (2004-12-25) [i586-linux-gnu]
>
> Aborted
>
> [gus@hibernatus Bifurcation]$ ls -ls test-mmap
> 4 -rw-rw-r--  1 gus gus 4096 Sep 10 07:45 test-mmap
> [gus@hibernatus Bifurcation]$ uname -a
> Linux hibernatus.gus.marcais.net 2.6.11-6mdksmp #1 SMP Tue Mar 22 15:40:42 
> CET 2005 i686 Pentium Pro unknown GNU/Linux
>
> The same behavior was showned on two different Linux machines (same 
> distribution though).
> Any idea why it crashes?

works fine for me:

   harp:~ > irb -r mmap -r fileutils
   irb(main):001:0> FileUtils::touch 'mmap'
   => ["mmap"]
   irb(main):002:0> mm = Mmap.new("mmap", "rw")
   => #<Mmap:0xb7570b3c>
   irb(main):003:0> 1.times { |i| p i; mm << ("\0" * 1024) }
   0
   => 1
   irb(main):004:0> 10.times { |i| p i; mm << ("\0" * 1024) }
   0
   1
   2
   3
   4
   5
   6
   7
   8
   9
   => 10
   irb(main):005:0> 20.times { |i| p i; mm << ("\0" * 1024) }
   0
   1
   2
   3
   4
   5
   6
   7
   8
   9
   10
   11
   12
   13
   14
   15
   16
   17
   18
   19
   => 20
   irb(main):006:0> mm
   => #<Mmap:0xb7570b3c>
   irb(main):007:0> mm.to_str.size
   => 31744
   irb(main):008:0> mm.msync
   => #<Mmap:0xb7570b3c>
   irb(main):009:0> IO::read('mmap').size
   => 31744


thoughts:

   - redhat has had many bugs with there mmap code.  a short while ago i
     reported a simple c program that would hang the kernel

       https://bugzilla.redhat.com/bugzilla/show_bug.cgi?id=124626

     perhaps your linux version is buggy.  if so bug the developers - mmap is
     such a fundemental operation (programs are always mmap'd when run by the
     kernel) that it's astounding that bugs like this get released.

   - did you install ruby via some package manager but compile the mmap dist?
     if you i'd (always) compile ruby vs. trusting the binary dists and compile
     mmap using this ruby, then see if the problem persists.

other than that i can tell you i'm abusing mmap in the most sinister ways with
no crashes or data corruption on a variety of linux dists.

hth.

-a
-- 
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